The present invention broadly relates to an Internet protocol multimedia network subsystem (IMS) and, more particularly relates to an IMS network by which subscriber user equipment (UE) registration is forked to multiple serving call session control functions (S-CSCFs).
An IMS is a standardized set of architecture specifications of the Third Generation Partnership Project (3GPP) that provides mobile and fixed multimedia services. IMS uses a Voice over Internet Protocol (VoIP) implementation that is backward compatible with existing phone systems that use packet-switched or circuit switched technologies. The basic principles of IMS and 3GPP are well known by those skilled in the art (see http://www.3gpp.org). IMS is structured to include private and public identification numbers for user equipment (UE). The public user identity (PUID) is analogous to the telephone number used in the old PSTN system, but in the format of a session initiation protocol (SIP) universal resource identity (URI) or a Tel URI format. Tel URI format is used for conventional E.164 numbers. The definitions and specifications for Tel URI and SIP URI can be found in RFC3966, 3261 and 2396.
The private user identity (PRID) is a unique global identity assigned by the network operator to identify the user associated with the user equipment (UE). The PRID is used exclusively for identification and authentication purposes (registration, authorization, administration and accounting functions in IMS) and not for routing of SIP requests to the user. Each user may be assigned one or more PRIDs and PUIDs. The PRID is in the form of a network access identifier (NAI) scheme, as defined in RFC2486 (http://rfc.net/rfc2486.html). As the IMS network uses session initiation protocol (SIP) for the registration process to admit a subscriber onto the network. The IMS network relies largely on the PRID for subscriber authentication and session establishment of a secured channel. Once the registration is completed, the serving call session control function (S-CSCF) establishes a predetermined relation between the public user ID (PUID) and the current location of the subscriber. As such, regardless of the particular location of the subscriber (i.e., the UE), either in its home network or in any other roaming network, the PUID can be used to call the subscriber (i.e., the UE) through network routing.
As such, session initiation protocol (SIP) registration of a conventional IP multimedia system (IMS) creates a single public user ID (PUID) that is bound to a single serving-call session control function (S-CSCF). As a result, if the UE-associated S-CSCF fails, no incoming calls or outgoing calls can be made to/from that UE until a “next” registration occurs. This results in call failures until the user registration occurs which will result in selection of a new S-CSCF.